1. Field of the Invention
The present invention relates to vibration control modules.
2. Background Information
Vibration control modules are used to isolate an element from a source of vibration. For example, integrated circuits are typically fabricated on a platform with photolithographic equipment. Relative movement between the platform and the light source must be kept at a minimum to insure accurate fabrication of the integrated circuits.
The platform is typically attached to a table that is located on a floor of a clean room. The floor has a natural vibrational tremor that can be transmitted into the table. Additionally, the integrated circuits are typically fabricated on a wafer. Each wafer is loaded onto the platform in a docking routine. Docking the wafer creates a shock load that is transmitted into the platform. The floor tremor and the shock load may cause relative displacement between the platform and the light source, thereby reducing the accuracy of the fabrication process.
U.S. Pat. No. 5,000,415 issued to Sandercock and assigned to the assignee of the present invention, Newport Corp., discloses a vibration isolator that has an active isolator assembly which actively isolates a load from a floor. The load may be the platform of a photolithographic table. The active isolator assembly includes a plurality of piezoelectric actuators that can vary the distance between the load and the floor surface to compensate for movement in the floor. For example, the floor may oscillate so that the floor surface moves toward the load and away from the load. When the floor moves toward the load the piezoelectric actuators contract so that the motion of the load relative to inertial space is reduced compared to that of the floor. Likewise, when the floor moves away from the load the actuators expand.
The active vibration isolator disclosed in the Sandercock patent includes a sensor that senses the movement of the floor and circuitry to provide a control loop to synchronize the contraction/expansion of the actuators with the movement in the floor. Sandercock also discloses the use of sensors which sense the velocity of the load to provide a feedback loop that is coupled to the feedforward loop.
The actuators are typically controlled by a linear control system that includes a controller such as a digital signal processor (DSP). The controller provides output signals to drive the actuators and isolate the load, in response to the output signals of the sensors. There may be situations where the floor disturbance causes the controller to provide a signal to move the actuators beyond the physical capabilities of the piezoelectric transducers. This event is typically referred to as saturation.
During saturation the load is displaced a value that is different from the expectations of the control system. The feedback is prone to self sustaining oscillations.
The problem of modifying linear control systems to account for actuator saturation without compromising performance under normal operations is typically referred to as xe2x80x9canti-windupxe2x80x9d. The term anti-windup is used because most linear control systems use a proportional plus integral plus derivative control scheme wherein the integrator would xe2x80x9cwindupxe2x80x9d to excessively large values during saturation.
FIG. 1 shows an experiment performed on a saturated table with a vibration control module that does not have an anti-windup feature. The plant input is the movement of the module actuators. The plant output is the movement of the load as sensed by the sensors. As shown, approximately 20 seconds transpired before the system stabilized from a saturation event. This 20 seconds can be critical if the load is a photolithographic table used to mass produce integrated circuits. Every second required to stabilize the table reduces the throughput of the manufacturing process.
One embodiment of the present invention is a vibration control module that includes a sensor that can sense a movement of an element and an actuator that can move the element. The sensor and actuator are coupled to a controller that performs a control routine wherein an output of a first anti-windup stage is summed with the output of the sensor. The summation is provided as an input to a nominal controller. The output of the nominal controller is provided to the actuator. The input to the first anti-windup stage is a summation of the output of the nominal controller and an actuator saturation level.